This invention relates to a gas sensor. In particular, this invention relates to a gas sensor provided on a semiconductor substrate.
Gas sensors are used in a number of different applications to sense the composition and/or concentration of various gases. One example application is in the field of supply chain monitoring, in which the levels of CO2 present in the air surrounding consumables such as food or beverages is monitored to determine suitability for consumption. The monitoring may typically be carried out at various stages in the distribution chain. Other applications include air quality monitoring, use in heating, ventilation and air conditioning (HVAC) system in buildings or automobiles, or CO2 monitoring in greenhouses.
FIG. 1 illustrates a first example of a known kind of gas sensor 20. The sensor 20 is provided on a semiconductor substrate 2 (typically silicon), and includes an elongate sensor element 4 provided in the form of a meander line. The sensor element 4 is terminated at either end by a pair of electrical contacts 10, allowing an electrical current to be passed through the sensor element 4 during operation. The sensor element 4 is situated on an upper surface of a bridge structure 6, which extends across an opening 8 in the substrate 2. The bridge structure 6 itself comprises a thin membrane fabricated by under-etching a portion of the surface of the substrate 2 to form the opening 8. As illustrated in FIG. 1, the sensor element 4 has an upper surface that is exposed to the surrounding environment, allowing the sensor element 4 to come into contact with a gas to be sensed.
Also provided on the substrate 2 is a heater. The heater is includes a resistive element 14 through which a current is passed via a pair of electrical terminals 12. The resistive element 14 in this example is also provided in the form of a meander line. The purpose of the heater 14 in this example is to compensate for changes in ambient temperature by acting as a reference resistance.
The gas sensor 20 operates as follows. The sensor is first brought into contact with a gas to be sensed. It is noted that the gas may in some examples comprise a mixture of constituents. In such examples, the gas sensor can be used to determine the composition of the gas by determining the relative concentrations of the constituents (a common example being the concentration of CO2 present in air).
To determine the concentration of the gas present, a current is passed through the sensor element 4 via the terminals 10. This causes the sensor element 4 to heat up. The rate at which heat can be carried away from the sensor element 4 by the surrounding gas is proportional to the thermal conductivity of the gas, which in turn is proportional to the concentration/composition of the gas. Accordingly, for a given gas concentration/composition, the heated sensor element 4 will reach thermal equilibrium at a certain corresponding temperature. This equilibrium temperature can be determined by measuring the resistance of the sensor element 4. In summary therefore, measurement of the resistance of the sensor element 4 can be used to determine the concentration/composition of gas that is in the vicinity of the sensor element 4.
As noted above, the sensor element 4 is provided in the form of a meander line. This increases the sensitivity of the gas sensor 20 by increasing the surface area of the sensor element 4 within the constraints of the space that is available for the sensor element 4 on the bridge structure 6. Nevertheless, the overall sensitivity of the gas sensor 20 is limited by the overall size and surface area of the sensor element 4 available for contact with the gas.
A second example of a gas sensor 30 is shown in FIGS. 2a and 2b. FIG. 2b shows a cross section of the gas sensor 30 through the line I in FIG. 2a. In this example, the gas sensor 30 includes a sensor element 34 located on a semiconductor substrate 2. The sensor element 34 comprises a metallic resistive element in the form of a meander line, and is produced using known metallisation techniques for semiconductor wafer processing. The formation of the sensor element 34 can be integrated with the formation of other metallisation features (e.g. power or signal lines) in the substrate 2 during manufacture. These additional features 32 are shown schematically below the sensor element 34 in FIGS. 2a and 2b. 
The operation of the gas sensor 30 shown in FIGS. 2a and 2b is much the same as that described above in relation to the example of FIG. 1.
As shown in FIGS. 2a and 2b, an area 38 of the substrate 2 corresponding to the centre of the meander line of the sensor element 34 has been removed by etching. In principle, this increases the sensitivity of the sensor element 34 by exposing the side walls thereof, thereby increasing the surface area of the sensor element 34 that is available for contact with the gas to be sensed. Nevertheless, the sensitivity of the gas sensor is still limited by the overall size and surface area of the sensor element 34.
JP 2005/003468 describes a flow sensor comprising a resistor supported at either end of a meander line arrangement.
U.S. Pat. No. 5,597,953 describes a gas moisture sensor having a window on which a tape heater in the shaped of a meander line is provided. The tape heater is coated with a moisture sensitive layer.
U.S. Pat. No. 4,349,808 and a paper by R. Batha et al. entitled “High-Tc air-bridge microbolometers fabricated by silicon micromachining technique” published in Microelectronic Engineering 27 (1995) p. 499-502, do not describe gas sensors, but instead relate to bolometers.
JP 6,118,046 describes an atmosphere sensor comprising a heating resistor arranged on a bridged thin film insulator. US 2004/251117 describes a suspended thin film resistor.
U.S. Pat. No. 5,753,916 describes a detector for an infrared gas analyzer, and U.S. Pat. No. 5,756,878 describes a thermal conductivity measuring device. Neither of these documents relate to a gas sensor of the kind provided on a semiconductor substrate.